1 00:00:00,000 --> 00:00:00,120 2 00:00:00,120 --> 00:00:02,470 The following content is provided under a Creative 3 00:00:02,470 --> 00:00:03,890 Commons license. 4 00:00:03,890 --> 00:00:06,920 Your support will help MIT OpenCourseWare continue to 5 00:00:06,920 --> 00:00:10,580 offer high quality educational resources for free. 6 00:00:10,580 --> 00:00:13,470 To make a donation or view additional materials from 7 00:00:13,470 --> 00:00:17,875 hundreds of MIT courses, visit MIT OpenCourseWare at 8 00:00:17,875 --> 00:00:20,410 ocw.mit.edu. 9 00:00:20,410 --> 00:00:26,470 GERALD SCHNEIDER: OK, well we didn't get too far with our 10 00:00:26,470 --> 00:00:29,680 classes on axon growth last time. 11 00:00:29,680 --> 00:00:32,850 We were talking about the growth cone. 12 00:00:32,850 --> 00:00:39,280 And I want next to talk about an experiment on axonal 13 00:00:39,280 --> 00:00:40,130 pathfinding. 14 00:00:40,130 --> 00:00:45,120 How do axons when they're elongating find their way to 15 00:00:45,120 --> 00:00:46,370 their target? 16 00:00:46,370 --> 00:00:52,580 17 00:00:52,580 --> 00:00:55,560 We'll see that the simple way of thinking about it is that 18 00:00:55,560 --> 00:01:01,580 they just follow some kind of chemical pathway all the way 19 00:01:01,580 --> 00:01:02,785 to their target. 20 00:01:02,785 --> 00:01:05,740 And that there's chemical specificity all along the way. 21 00:01:05,740 --> 00:01:10,420 And that was just that was the hypothesis of Roger Sperry. 22 00:01:10,420 --> 00:01:14,830 He called it the chemoaffinity or chemospecificity 23 00:01:14,830 --> 00:01:16,080 hypothesis. 24 00:01:16,080 --> 00:01:18,810 25 00:01:18,810 --> 00:01:23,980 It's been largely verified, but not in the way Sperry 26 00:01:23,980 --> 00:01:25,230 envisioned. 27 00:01:25,230 --> 00:01:26,890 28 00:01:26,890 --> 00:01:30,800 So let me talk a little bit about that. 29 00:01:30,800 --> 00:01:34,750 First of all, the grasshopper leg gave an interesting 30 00:01:34,750 --> 00:01:37,770 example of how axons can find their target. 31 00:01:37,770 --> 00:01:40,720 32 00:01:40,720 --> 00:01:49,650 Here at the top, imagine a grasshopper leg at a very 33 00:01:49,650 --> 00:01:51,070 early stage of development. 34 00:01:51,070 --> 00:01:53,940 So it's sort of a limb bud there. 35 00:01:53,940 --> 00:02:00,370 And imagine that it's transparent and we can see the 36 00:02:00,370 --> 00:02:05,940 peripheral neurons sending our axon out, growling towards the 37 00:02:05,940 --> 00:02:07,342 central ganglion here. 38 00:02:07,342 --> 00:02:10,300 39 00:02:10,300 --> 00:02:13,600 And it's found that they always 40 00:02:13,600 --> 00:02:15,570 follow a certain pathway. 41 00:02:15,570 --> 00:02:23,760 They've shown there in the light pink the envelope of all 42 00:02:23,760 --> 00:02:26,830 the growth of all the axons of a bunch of grasshoppers. 43 00:02:26,830 --> 00:02:30,660 And you can see they always have certain bends. 44 00:02:30,660 --> 00:02:34,130 They always making a sharp bend here. 45 00:02:34,130 --> 00:02:37,330 And then they turn again towards the central ganglion. 46 00:02:37,330 --> 00:02:40,700 So what is guiding it? 47 00:02:40,700 --> 00:02:47,100 Well it turns out there are nonneuronal cells along the 48 00:02:47,100 --> 00:02:50,430 way, sort of like a glial cell. 49 00:02:50,430 --> 00:02:54,750 50 00:02:54,750 --> 00:03:02,210 The hypothesis was soon proposed that they're using 51 00:03:02,210 --> 00:03:06,440 those as guides, that these are guidepost cells. 52 00:03:06,440 --> 00:03:07,700 So how could you test that? 53 00:03:07,700 --> 00:03:11,300 Well very simply. 54 00:03:11,300 --> 00:03:19,950 Before the first axons ever get here, zap them with a 55 00:03:19,950 --> 00:03:22,800 laser beam. 56 00:03:22,800 --> 00:03:27,760 And they could identify these same cells, both the neurons 57 00:03:27,760 --> 00:03:31,970 and these peripheral so-called guidepost cells. 58 00:03:31,970 --> 00:03:36,170 They could identify them from one grasshopper to the next. 59 00:03:36,170 --> 00:03:42,600 And so it's a big advantage to using these simpler animals 60 00:03:42,600 --> 00:03:45,230 where rather than dealing with whole populations of cells, 61 00:03:45,230 --> 00:03:48,450 which you'd have to do with mammals and other larger 62 00:03:48,450 --> 00:03:51,970 animals, you can sometimes find individually identified 63 00:03:51,970 --> 00:03:53,460 themselves. 64 00:03:53,460 --> 00:03:56,820 And in fact, these cells have names. 65 00:03:56,820 --> 00:04:02,400 Well when they zap these cells, here's the control. 66 00:04:02,400 --> 00:04:05,030 And without those cells there, look what happens. 67 00:04:05,030 --> 00:04:10,820 The axons simply start splaying out, forming multiple 68 00:04:10,820 --> 00:04:13,480 branches, they don't seem to know where to go after they 69 00:04:13,480 --> 00:04:17,529 get to the previous cell. 70 00:04:17,529 --> 00:04:20,032 So that it was proposed then that these were guidepost 71 00:04:20,032 --> 00:04:23,020 cells and the pathway is formed. 72 00:04:23,020 --> 00:04:24,270 What would be happening? 73 00:04:24,270 --> 00:04:26,900 74 00:04:26,900 --> 00:04:31,760 Let's say the axon has gotten to this point. 75 00:04:31,760 --> 00:04:34,700 What is it doing to reach to the next point? 76 00:04:34,700 --> 00:04:38,470 How does it to know to go here rather than to turn around and 77 00:04:38,470 --> 00:04:42,020 go back the other, or to go out towards the surface, or go 78 00:04:42,020 --> 00:04:44,440 down to the other edge of the leg? 79 00:04:44,440 --> 00:04:48,260 The idea is that the philipodia can be very long. 80 00:04:48,260 --> 00:04:50,710 They can up to 200 microns. 81 00:04:50,710 --> 00:04:54,800 And these legs are very small early in development. 82 00:04:54,800 --> 00:05:00,150 So they send out these long philipodia and retract them. 83 00:05:00,150 --> 00:05:06,440 And if they can adhere to one of these guidepost cells, they 84 00:05:06,440 --> 00:05:08,450 could move towards it, literally be 85 00:05:08,450 --> 00:05:10,630 pulled towards it. 86 00:05:10,630 --> 00:05:14,520 And then by a metabolic change after they reach there, they 87 00:05:14,520 --> 00:05:19,170 could start the same process again and go on to the next 88 00:05:19,170 --> 00:05:19,890 guidepost cell. 89 00:05:19,890 --> 00:05:23,530 So it would be a sort of connect the dots type of 90 00:05:23,530 --> 00:05:24,780 guidance system. 91 00:05:24,780 --> 00:05:28,510 92 00:05:28,510 --> 00:05:33,040 It's the pioneering axon that uses those cells and then 93 00:05:33,040 --> 00:05:37,370 later growing axons seem to follow the first one. 94 00:05:37,370 --> 00:05:40,880 95 00:05:40,880 --> 00:05:44,480 Well we studied the mammalian optic track, several people in 96 00:05:44,480 --> 00:05:47,980 my laboratory, a few years back. 97 00:05:47,980 --> 00:05:51,520 And we found that what happens in the grasshopper leg is not 98 00:05:51,520 --> 00:05:53,710 what happens in the mammalian optic track. 99 00:05:53,710 --> 00:05:56,860 100 00:05:56,860 --> 00:06:02,700 The initial pioneering axons grow out, now this is after 101 00:06:02,700 --> 00:06:04,630 the optic chiasm. 102 00:06:04,630 --> 00:06:08,400 They grow up in a wide swath, the full width of the optic 103 00:06:08,400 --> 00:06:11,020 track, and they space themselves. 104 00:06:11,020 --> 00:06:13,580 They space out, even though there's not very many of them 105 00:06:13,580 --> 00:06:15,880 when they first grow. 106 00:06:15,880 --> 00:06:19,330 And they grow up the side of the [UNINTELLIGIBLE] brain, 107 00:06:19,330 --> 00:06:21,230 towards the geniculate body. 108 00:06:21,230 --> 00:06:26,200 And then the later growing ones just fit in between. 109 00:06:26,200 --> 00:06:29,790 And they don't form any noticeable fascicles where 110 00:06:29,790 --> 00:06:33,420 axons could be following each other until much later on. 111 00:06:33,420 --> 00:06:36,020 112 00:06:36,020 --> 00:06:38,330 So it's not like the grasshopper. 113 00:06:38,330 --> 00:06:40,230 So we know that there are other 114 00:06:40,230 --> 00:06:41,965 mechanisms of axon guidance. 115 00:06:41,965 --> 00:06:45,270 116 00:06:45,270 --> 00:06:47,830 Let's go over another experiment on axon growth. 117 00:06:47,830 --> 00:06:49,010 This is a very interesting one. 118 00:06:49,010 --> 00:06:52,590 It's often republished and it's described in the Purves 119 00:06:52,590 --> 00:06:54,440 and Lichtman reading. 120 00:06:54,440 --> 00:06:56,610 I give you the page numbers there. 121 00:06:56,610 --> 00:06:58,850 What did he do? 122 00:06:58,850 --> 00:07:03,690 Well Mauthner cells are these giant neurons in the hind 123 00:07:03,690 --> 00:07:08,170 brain of some fish and amphibians. 124 00:07:08,170 --> 00:07:10,510 And you can find them in growing tadpoles. 125 00:07:10,510 --> 00:07:15,150 126 00:07:15,150 --> 00:07:23,070 At the top you a picture of the hind brain there. 127 00:07:23,070 --> 00:07:28,980 And I've outlined with the red dash line there, the piece of 128 00:07:28,980 --> 00:07:39,980 hind brain that Hillard cut out and then he put it back. 129 00:07:39,980 --> 00:07:44,460 And the top row there, he put them back in the same 130 00:07:44,460 --> 00:07:45,090 orientation. 131 00:07:45,090 --> 00:07:47,790 He just kind of it loose, looked at it out, 132 00:07:47,790 --> 00:07:49,800 stuck it back in. 133 00:07:49,800 --> 00:07:52,530 Well you can see, the axons grew out normally. 134 00:07:52,530 --> 00:07:57,220 But in a few times they actually didn't get on the 135 00:07:57,220 --> 00:07:59,400 wrong side of the brain. 136 00:07:59,400 --> 00:08:02,600 Here's one that rather than crossing stayed ipsilarily. 137 00:08:02,600 --> 00:08:04,965 But otherwise, its trajectory was normal. 138 00:08:04,965 --> 00:08:08,270 139 00:08:08,270 --> 00:08:10,940 There could be mechanical effects that were affecting 140 00:08:10,940 --> 00:08:13,120 the tissue destruction that had affected the 141 00:08:13,120 --> 00:08:15,400 guidance of the axon. 142 00:08:15,400 --> 00:08:17,350 That's not certain. 143 00:08:17,350 --> 00:08:21,100 The interesting thing is, what happens if when he takes that 144 00:08:21,100 --> 00:08:26,590 piece out, he turns it 180 degrees and then puts it back? 145 00:08:26,590 --> 00:08:30,300 So now if they follow the course determined by where 146 00:08:30,300 --> 00:08:33,679 they came from, [INAUDIBLE] grow in the wrong direction. 147 00:08:33,679 --> 00:08:37,830 And you can see in every case here, the axon when it grows 148 00:08:37,830 --> 00:08:39,799 out grows [UNINTELLIGIBLE] 149 00:08:39,799 --> 00:08:41,850 instead of caudally?. 150 00:08:41,850 --> 00:08:48,210 It appears is to be getting a cue from the original tissue. 151 00:08:48,210 --> 00:08:53,000 But then when it crosses into the tissue that didn't get 152 00:08:53,000 --> 00:08:57,320 turned around, it turns around. 153 00:08:57,320 --> 00:09:02,840 There seems to be a substrate cue of some sort. 154 00:09:02,840 --> 00:09:05,330 So you can say, well then why doesn't it start going just 155 00:09:05,330 --> 00:09:06,580 back and forth back and forth? 156 00:09:06,580 --> 00:09:08,740 It doesn't do that. 157 00:09:08,740 --> 00:09:14,130 By the time it's turned around and grown caudally, its 158 00:09:14,130 --> 00:09:16,560 response to these cues has changed. 159 00:09:16,560 --> 00:09:19,300 And now it continues caudally. 160 00:09:19,300 --> 00:09:21,710 And it ends up farming fairly normal 161 00:09:21,710 --> 00:09:23,890 connections of Mauthner cells. 162 00:09:23,890 --> 00:09:25,980 It just follows an abnormal trajectory. 163 00:09:25,980 --> 00:09:32,920 164 00:09:32,920 --> 00:09:36,805 So at the time the Purves and Lichtman book was written a 165 00:09:36,805 --> 00:09:41,210 few years back, this was 1985 when they published it, but 166 00:09:41,210 --> 00:09:45,550 they could see these four types of mechanisms. 167 00:09:45,550 --> 00:09:50,770 And I put in bold there the ones that have been verified 168 00:09:50,770 --> 00:09:56,710 in other experiments in a number of different species as 169 00:09:56,710 --> 00:10:00,040 valid mechanisms of axon guidance. 170 00:10:00,040 --> 00:10:03,590 Sterotropism means the shape of the surface that it's 171 00:10:03,590 --> 00:10:06,690 growing on. 172 00:10:06,690 --> 00:10:09,800 So for example, if I did an early brain lesion, which I've 173 00:10:09,800 --> 00:10:12,600 done a lot of in the developing hamster when the 174 00:10:12,600 --> 00:10:17,450 optic track was first forming, I could produce tissue 175 00:10:17,450 --> 00:10:21,520 disruptions that would deflect the axons towards the surface 176 00:10:21,520 --> 00:10:23,425 and they would grow abnormally. 177 00:10:23,425 --> 00:10:27,450 178 00:10:27,450 --> 00:10:29,670 That's an example of this theory of tropism. 179 00:10:29,670 --> 00:10:35,500 I've left the same tissue there, I've just changed the 180 00:10:35,500 --> 00:10:39,420 membrane surfaces that these axons are following. 181 00:10:39,420 --> 00:10:43,680 The second one they called galvanotropism because of the 182 00:10:43,680 --> 00:10:47,382 evidence that slight very small electric currents in 183 00:10:47,382 --> 00:10:50,770 tissues can affect the way axons grow. 184 00:10:50,770 --> 00:10:54,010 And there is evidence for that. 185 00:10:54,010 --> 00:10:59,010 The problem is that the currents that actually exist 186 00:10:59,010 --> 00:11:02,390 in developing tissue are extremely tiny. 187 00:11:02,390 --> 00:11:04,810 And if they are having effects, they don't appear to 188 00:11:04,810 --> 00:11:09,550 be mainly on these long trajectories of axons. 189 00:11:09,550 --> 00:11:11,200 They may have local effects. 190 00:11:11,200 --> 00:11:18,120 And there haven't been many studies of that, a few in 191 00:11:18,120 --> 00:11:19,650 these early years. 192 00:11:19,650 --> 00:11:22,320 But then as more and more evidence for chemical cues 193 00:11:22,320 --> 00:11:26,040 developed and the methods improved for studying various 194 00:11:26,040 --> 00:11:30,620 molecular cues, the emphasis changed. 195 00:11:30,620 --> 00:11:33,190 The only chemical guidance was based on 196 00:11:33,190 --> 00:11:34,440 differential adhesion. 197 00:11:34,440 --> 00:11:36,890 198 00:11:36,890 --> 00:11:42,540 We've mentioned how philipodia have cell adhesion molecules 199 00:11:42,540 --> 00:11:46,120 in their membranes, especially at their tips. 200 00:11:46,120 --> 00:11:50,020 And they will adhere to other cell adhesion molecules. 201 00:11:50,020 --> 00:11:53,360 And that can guide the axon. 202 00:11:53,360 --> 00:11:57,440 So more adhesive substrates will effect axons differently 203 00:11:57,440 --> 00:11:59,370 than the less adhesive substrates. 204 00:11:59,370 --> 00:12:04,920 And if there's no adhesion at all, then the axons actually 205 00:12:04,920 --> 00:12:06,750 have trouble growing at all. 206 00:12:06,750 --> 00:12:09,510 They need some adhesion to be able to grow. 207 00:12:09,510 --> 00:12:14,660 And then finally, chemical effects, chemotropism. 208 00:12:14,660 --> 00:12:21,460 And chemotropic mechanisms can be either by membrane 209 00:12:21,460 --> 00:12:25,840 molecules, so it requires contact of the membrane, or 210 00:12:25,840 --> 00:12:29,460 they can be chemicals that diffuse through the 211 00:12:29,460 --> 00:12:30,900 environment. 212 00:12:30,900 --> 00:12:36,720 And this was known too way back in '85 when Purves and 213 00:12:36,720 --> 00:12:39,340 Lichtman summarized developmental knowledge at 214 00:12:39,340 --> 00:12:40,590 that point. 215 00:12:40,590 --> 00:12:42,600 216 00:12:42,600 --> 00:12:48,020 Well since that time, people have clearly distinguished 217 00:12:48,020 --> 00:12:49,710 four types of chemical guidance. 218 00:12:49,710 --> 00:12:51,195 And it's very clear. 219 00:12:51,195 --> 00:12:53,920 220 00:12:53,920 --> 00:12:57,460 This was a picture published in a review paper that 221 00:12:57,460 --> 00:12:58,820 summarized them. 222 00:12:58,820 --> 00:13:03,820 There are long range cues that involve diffusing molecules. 223 00:13:03,820 --> 00:13:07,820 And then the short range cues that require content. 224 00:13:07,820 --> 00:13:11,090 And the results can be of two types in both cases. 225 00:13:11,090 --> 00:13:14,620 They can be attraction and they can be repulsion of 226 00:13:14,620 --> 00:13:19,470 various sorts, like barriers to growth or 227 00:13:19,470 --> 00:13:21,400 inhibition of growth. 228 00:13:21,400 --> 00:13:24,960 And they list some of the molecules there, some of which 229 00:13:24,960 --> 00:13:32,920 we will take up again a little later today or the next time. 230 00:13:32,920 --> 00:13:35,270 So we have attraction and repulsion. 231 00:13:35,270 --> 00:13:39,410 And repulsion can also mean inhibition of growth. 232 00:13:39,410 --> 00:13:42,230 And these can involve effects of either defusing chemicals 233 00:13:42,230 --> 00:13:44,830 or effects of contact. 234 00:13:44,830 --> 00:13:48,730 So to go to the attraction effects first, we have 235 00:13:48,730 --> 00:13:50,240 cell-cell adhesion. 236 00:13:50,240 --> 00:13:52,080 Of course, this requires memory contact. 237 00:13:52,080 --> 00:13:54,920 These are the CAMs, the cell adhesion molecules. 238 00:13:54,920 --> 00:13:57,690 239 00:13:57,690 --> 00:14:02,710 Extracellular or matrix molecules can be adhesive as 240 00:14:02,710 --> 00:14:06,290 well, like the laminins and the cadherins. 241 00:14:06,290 --> 00:14:11,110 We can follow laminin rich tracks that axons follow in 242 00:14:11,110 --> 00:14:14,780 development, for example. 243 00:14:14,780 --> 00:14:19,610 And they are growth factors that increase growth upon 244 00:14:19,610 --> 00:14:22,770 contact or they act by diffusion. 245 00:14:22,770 --> 00:14:25,580 The contact molecules, many of them are called 246 00:14:25,580 --> 00:14:33,860 [UNINTELLIGIBLE], the diffusable ones are like nerve 247 00:14:33,860 --> 00:14:36,130 growth factors. 248 00:14:36,130 --> 00:14:39,590 And other members of that family of growth factors, the 249 00:14:39,590 --> 00:14:42,900 neurotrophins nerve growth factor was the first of 250 00:14:42,900 --> 00:14:46,490 several members of that same family. 251 00:14:46,490 --> 00:14:50,660 The netrins are a molecule we'll talk about a minute. 252 00:14:50,660 --> 00:14:53,380 And there's several different families of growth factors 253 00:14:53,380 --> 00:14:56,350 that have been discovered. 254 00:14:56,350 --> 00:14:58,610 So the other effect, the inhibition of growth, first of 255 00:14:58,610 --> 00:15:00,210 all, we have midline barriers. 256 00:15:00,210 --> 00:15:05,150 For example, in the superior colliculus, axons from one eye 257 00:15:05,150 --> 00:15:08,970 in an animal were almost all the axons are crossed, axons 258 00:15:08,970 --> 00:15:12,140 from one eye will go to one site, axons from the other eye 259 00:15:12,140 --> 00:15:15,150 will go to the other side. 260 00:15:15,150 --> 00:15:20,960 And why don't they cross the midline? 261 00:15:20,960 --> 00:15:24,020 There appears to be a barrier there. 262 00:15:24,020 --> 00:15:30,140 And to test that, in my lab we've actually cut the radial 263 00:15:30,140 --> 00:15:33,832 cells, or glial cells, that are found at the midline. 264 00:15:33,832 --> 00:15:37,790 With a little cut we caused the process that goes up to 265 00:15:37,790 --> 00:15:39,540 the surface to degenerate. 266 00:15:39,540 --> 00:15:42,690 And what happens is that the axons will spread across the 267 00:15:42,690 --> 00:15:44,884 midline if you do that. 268 00:15:44,884 --> 00:15:48,370 And if you remove their competition from the eye, they 269 00:15:48,370 --> 00:15:55,570 will spread right across both sides of the optic tectum. 270 00:15:55,570 --> 00:15:57,830 If you don't do that damage at the midline, 271 00:15:57,830 --> 00:15:59,390 they won't do that. 272 00:15:59,390 --> 00:16:01,190 So there's definitely a midline barrier. 273 00:16:01,190 --> 00:16:03,900 274 00:16:03,900 --> 00:16:07,150 And it's now known that midline radial glia actually 275 00:16:07,150 --> 00:16:14,160 secrete cardioglycans and these appear to be responsible 276 00:16:14,160 --> 00:16:17,310 for that barrier. 277 00:16:17,310 --> 00:16:19,720 There are factors in the oligodendrocytes. 278 00:16:19,720 --> 00:16:23,150 Do you remember what oligodendrocytes are? 279 00:16:23,150 --> 00:16:27,040 They're glial cells in the central nervous system. 280 00:16:27,040 --> 00:16:29,285 They act like the Schwann cells in the peripheral 281 00:16:29,285 --> 00:16:32,980 nervous system in that they form myelin. 282 00:16:32,980 --> 00:16:37,430 Their membranes are wrapped around the membrane of axons, 283 00:16:37,430 --> 00:16:38,870 wrapped right around the axon. 284 00:16:38,870 --> 00:16:41,880 285 00:16:41,880 --> 00:16:49,320 Well when the oligodendrocytes appear in development, at a 286 00:16:49,320 --> 00:16:54,960 very early stage in their membranes, there is a molecule 287 00:16:54,960 --> 00:17:01,540 that appears that has been named Nogo by the discoverer 288 00:17:01,540 --> 00:17:06,040 in Switzerland because it inhibits axon growth. 289 00:17:06,040 --> 00:17:10,140 290 00:17:10,140 --> 00:17:14,160 And it's been proposed it's a major factor that prevents 291 00:17:14,160 --> 00:17:16,510 some axon regeneration. 292 00:17:16,510 --> 00:17:21,089 Although, the research that we and others have done has not 293 00:17:21,089 --> 00:17:24,000 supported that. 294 00:17:24,000 --> 00:17:27,069 And then there's various secreted and transmembrane 295 00:17:27,069 --> 00:17:29,725 proteins that also inhibit growth. 296 00:17:29,725 --> 00:17:32,740 And we'll see some of that now. 297 00:17:32,740 --> 00:17:35,680 When we talk about these effects, I want you to be able 298 00:17:35,680 --> 00:17:40,680 to distinguish trophic effects and tropic effects. 299 00:17:40,680 --> 00:17:45,510 300 00:17:45,510 --> 00:17:50,240 One word has the h, one does not. 301 00:17:50,240 --> 00:17:55,790 Tropic means it influences the direction the axon is growing. 302 00:17:55,790 --> 00:17:56,860 That's what tropic meant. 303 00:17:56,860 --> 00:18:02,600 A tropism is something that affects direction. 304 00:18:02,600 --> 00:18:09,160 In animal behavior we talked about thigmotropism, for 305 00:18:09,160 --> 00:18:13,010 example, an animal that tends to follow a surface, like it 306 00:18:13,010 --> 00:18:19,260 follows the wall in the dark, his direction is controlled by 307 00:18:19,260 --> 00:18:21,280 these surfaces. 308 00:18:21,280 --> 00:18:24,270 So tropic means influencing the direction of growth. 309 00:18:24,270 --> 00:18:28,510 Whereas trophic can mean two kinds of things. 310 00:18:28,510 --> 00:18:35,170 Survival promotion, so without trophic molecules, cells, at 311 00:18:35,170 --> 00:18:39,030 least at a certain station in their development, will die. 312 00:18:39,030 --> 00:18:42,090 They need trophic molecules to survive. 313 00:18:42,090 --> 00:18:44,700 But trophic also means promoting growth. 314 00:18:44,700 --> 00:18:51,510 So more growth factor will make them more vigorous in the 315 00:18:51,510 --> 00:18:55,100 growth of their axon and make those axons have more ability 316 00:18:55,100 --> 00:18:56,841 to compete. 317 00:18:56,841 --> 00:19:00,770 And we can give some examples. 318 00:19:00,770 --> 00:19:04,390 But first let me give one caveat here. 319 00:19:04,390 --> 00:19:07,830 The discovery by Mu-ming Poo's group that these guidance 320 00:19:07,830 --> 00:19:09,365 mechanisms aren't fixed. 321 00:19:09,365 --> 00:19:12,518 322 00:19:12,518 --> 00:19:16,220 He published a paper, the first one in a group of 323 00:19:16,220 --> 00:19:21,310 papers, showing that a molecule can change from 324 00:19:21,310 --> 00:19:27,060 having repulsive effects to attraction effects just by 325 00:19:27,060 --> 00:19:29,290 adding cyclic nucleotides. 326 00:19:29,290 --> 00:19:33,980 So for example, I'm sorry the picture is, let me blow it up 327 00:19:33,980 --> 00:19:36,170 there because I'm only interested here in these 328 00:19:36,170 --> 00:19:37,420 pictures at the top. 329 00:19:37,420 --> 00:19:46,370 330 00:19:46,370 --> 00:19:48,430 OK, in A here. 331 00:19:48,430 --> 00:19:53,730 He's using some semaphorin 3 that's diffusing out of a 332 00:19:53,730 --> 00:19:56,980 little pipette that he's put up here. 333 00:19:56,980 --> 00:20:00,270 And here is the axon growing. 334 00:20:00,270 --> 00:20:02,040 And to see what it does? 335 00:20:02,040 --> 00:20:03,370 It starts turning away. 336 00:20:03,370 --> 00:20:05,620 The only thing that's happening there is there's a 337 00:20:05,620 --> 00:20:10,760 diffusion of semaphorin 3 from that pipette shown by where 338 00:20:10,760 --> 00:20:12,220 the arrowhead is there. 339 00:20:12,220 --> 00:20:15,400 The axon is turning away. 340 00:20:15,400 --> 00:20:23,070 So now he's adding cyclic GMP. 341 00:20:23,070 --> 00:20:30,610 It's actually a agonistic cyclic GMP. 342 00:20:30,610 --> 00:20:32,670 It acts like cyclic GMP. 343 00:20:32,670 --> 00:20:34,545 And look what happens. 344 00:20:34,545 --> 00:20:39,100 It's the same experiment, he just adds that molecule. 345 00:20:39,100 --> 00:20:42,950 Now the axon is turning towards. 346 00:20:42,950 --> 00:20:46,870 Now it's going up the gradient instead of 347 00:20:46,870 --> 00:20:48,660 getting away from it. 348 00:20:48,660 --> 00:20:50,670 And he's plotted the quantitative results. 349 00:20:50,670 --> 00:20:53,300 You can see there's some variability, but it's turning 350 00:20:53,300 --> 00:20:55,240 away here and turning towards here. 351 00:20:55,240 --> 00:21:04,630 352 00:21:04,630 --> 00:21:10,690 He found that cyclic GMP or cyclic AMP, either one could 353 00:21:10,690 --> 00:21:14,670 be used and he could get these kinds of effects. 354 00:21:14,670 --> 00:21:21,180 This is pretty important as you'll see in a minute. 355 00:21:21,180 --> 00:21:25,870 Let's first see how that kind of finding on semaphorin 3, 356 00:21:25,870 --> 00:21:29,580 which was originally named collapsin. 357 00:21:29,580 --> 00:21:32,550 Why do you think it was called collapsin initially? 358 00:21:32,550 --> 00:21:36,670 Its first effect discovered was an inhibition of growth. 359 00:21:36,670 --> 00:21:40,340 And when axons were exposed to the molecule, you got that 360 00:21:40,340 --> 00:21:43,240 phenomenon we saw in the video clip the other day, growth 361 00:21:43,240 --> 00:21:46,390 cones would collapse, pull back, and change direction 362 00:21:46,390 --> 00:21:48,193 before they started growing again. 363 00:21:48,193 --> 00:21:54,170 364 00:21:54,170 --> 00:21:59,190 Well it was found that semaphorin was found 365 00:21:59,190 --> 00:22:04,390 especially in the ventral part of the developing spinal cord, 366 00:22:04,390 --> 00:22:07,260 not in the dorsal horn. 367 00:22:07,260 --> 00:22:10,590 That led to the proposal that these axons of the dorsal 368 00:22:10,590 --> 00:22:14,965 root, coming from the dorsal root ganglion cells, when they 369 00:22:14,965 --> 00:22:18,360 grew in, acted differently because they had a different 370 00:22:18,360 --> 00:22:19,610 response to semaphorin 3. 371 00:22:19,610 --> 00:22:22,240 372 00:22:22,240 --> 00:22:32,340 The largest axons, these, that were coming from sensory 373 00:22:32,340 --> 00:22:37,190 fibers innervating muscle, they were the axons that 374 00:22:37,190 --> 00:22:38,720 formed the monosynaptic reflex. 375 00:22:38,720 --> 00:22:41,480 So they went down in the ventral horn to 376 00:22:41,480 --> 00:22:44,220 contact motor neurons. 377 00:22:44,220 --> 00:22:48,100 They were not repelled by the semaphorin 3. 378 00:22:48,100 --> 00:22:54,830 But the smaller axons that terminated in the dorsal horn, 379 00:22:54,830 --> 00:22:58,780 some of them, actually others would terminate here in the 380 00:22:58,780 --> 00:23:02,030 intermediate region, but never going into the ventral horn. 381 00:23:02,030 --> 00:23:07,110 So the idea was that they were the ones responding to 382 00:23:07,110 --> 00:23:08,640 semaphorin 3. 383 00:23:08,640 --> 00:23:11,043 And there's been some verification of that idea. 384 00:23:11,043 --> 00:23:14,850 385 00:23:14,850 --> 00:23:18,660 Let's leave the spinal cord here for a minute and talk 386 00:23:18,660 --> 00:23:23,520 about the axons that terminate in the dorsal horn on 387 00:23:23,520 --> 00:23:24,990 secondary sensory neurons. 388 00:23:24,990 --> 00:23:28,120 And then the axon of the secondary sensory cell 389 00:23:28,120 --> 00:23:35,950 projects down across the midline and then ascends the 390 00:23:35,950 --> 00:23:38,680 cord on the opposite side, like this is the 391 00:23:38,680 --> 00:23:39,890 spinothalamic tract. 392 00:23:39,890 --> 00:23:41,945 That's how the spinothalamic tract originates. 393 00:23:41,945 --> 00:23:44,710 394 00:23:44,710 --> 00:23:48,770 Why do you think they grown down to the floor plate? 395 00:23:48,770 --> 00:23:52,200 Now when they're first doing this, you didn't have this 396 00:23:52,200 --> 00:23:54,200 thick area here. 397 00:23:54,200 --> 00:23:57,944 398 00:23:57,944 --> 00:24:00,910 But the floor plate was still there below the ventricle. 399 00:24:00,910 --> 00:24:06,810 400 00:24:06,810 --> 00:24:09,720 This was the work of Tom Jessell and his coworkers at 401 00:24:09,720 --> 00:24:10,800 Columbia University. 402 00:24:10,800 --> 00:24:14,900 He discovered molecules they called the netrin molecules 403 00:24:14,900 --> 00:24:18,210 that diffused from the floor plate region that specifically 404 00:24:18,210 --> 00:24:24,410 affect those axons from the dorsal horn cells, the 405 00:24:24,410 --> 00:24:27,110 secondary sensory cells, that form the spinothalamic tract. 406 00:24:27,110 --> 00:24:29,720 407 00:24:29,720 --> 00:24:32,310 They have tropic effects on those axons. 408 00:24:32,310 --> 00:24:35,800 409 00:24:35,800 --> 00:24:39,420 But if they're attracted to the floor plate, that's what 410 00:24:39,420 --> 00:24:46,030 gets them down there, why do they cross over and go way out 411 00:24:46,030 --> 00:24:50,570 to the lateral edge and then start ascending? 412 00:24:50,570 --> 00:24:53,450 In other words, if they were being attracted to the floor 413 00:24:53,450 --> 00:24:55,130 plate, why don't they get to the floor 414 00:24:55,130 --> 00:24:56,390 plate and stay there? 415 00:24:56,390 --> 00:24:59,930 416 00:24:59,930 --> 00:25:04,610 Since they don't, we have to assume that a metabolic change 417 00:25:04,610 --> 00:25:09,320 of the sort that Mu-ming Poo and his collaborators 418 00:25:09,320 --> 00:25:11,450 discovered must be acting. 419 00:25:11,450 --> 00:25:15,440 420 00:25:15,440 --> 00:25:19,130 At least conceptually we can understand what's happening. 421 00:25:19,130 --> 00:25:21,050 Even though some of the molecular 422 00:25:21,050 --> 00:25:23,215 mechanisms may not be known. 423 00:25:23,215 --> 00:25:25,140 We've discovered the type of mechanism 424 00:25:25,140 --> 00:25:26,390 that would be needed. 425 00:25:26,390 --> 00:25:30,170 426 00:25:30,170 --> 00:25:34,250 So we know that developing axons have to do a lot more 427 00:25:34,250 --> 00:25:37,320 than simply follow a path and then terminate 428 00:25:37,320 --> 00:25:38,730 when they get there. 429 00:25:38,730 --> 00:25:42,490 They're guided all along the way. 430 00:25:42,490 --> 00:25:46,660 We also know though that they change their mode of growth 431 00:25:46,660 --> 00:25:48,550 when they get into their terminal region. 432 00:25:48,550 --> 00:25:54,530 So I want to take a little time now to discuss that. 433 00:25:54,530 --> 00:25:56,960 Because we know that when they're elongating they grow 434 00:25:56,960 --> 00:25:59,540 about 10 times faster than they grow when they enter 435 00:25:59,540 --> 00:26:02,140 their terminal region, something changes. 436 00:26:02,140 --> 00:26:07,240 We also know that when their elongating they not only grow 437 00:26:07,240 --> 00:26:11,750 faster, but they tend to grow in vesicles. 438 00:26:11,750 --> 00:26:13,550 The later ones tend to vesiculate 439 00:26:13,550 --> 00:26:16,410 with the earlier ones. 440 00:26:16,410 --> 00:26:19,180 We also know that when they enter the terminal region and 441 00:26:19,180 --> 00:26:21,860 start growing more slowly, they start branching. 442 00:26:21,860 --> 00:26:23,760 That's how a terminal arbor forms. 443 00:26:23,760 --> 00:26:26,335 They don't branch like that when they're elongating. 444 00:26:26,335 --> 00:26:28,850 445 00:26:28,850 --> 00:26:34,245 So we studied this in the optic tract. 446 00:26:34,245 --> 00:26:38,100 And some of the work was Golgi study, some of it was filling 447 00:26:38,100 --> 00:26:41,365 of axons with various molecules. 448 00:26:41,365 --> 00:26:44,920 449 00:26:44,920 --> 00:26:49,300 The upper two pictures were from the Golgi work. 450 00:26:49,300 --> 00:26:51,510 And we found that through earlier in development we 451 00:26:51,510 --> 00:26:55,270 could always see the growth cone at the tip. 452 00:26:55,270 --> 00:26:58,880 And some of those, sorry, they were a couple of other methods 453 00:26:58,880 --> 00:27:03,030 not Golgi, but they were Golgi like in the result. 454 00:27:03,030 --> 00:27:08,030 Some of it was a molecule called DiI that would diffuse 455 00:27:08,030 --> 00:27:10,290 through the membrane of the cells. 456 00:27:10,290 --> 00:27:14,290 And we found that they were characterized by these 457 00:27:14,290 --> 00:27:17,520 periodic swellings. 458 00:27:17,520 --> 00:27:20,940 And also by lots of little philipodia. 459 00:27:20,940 --> 00:27:27,110 So the philipodia are not only at the growth cone, they're 460 00:27:27,110 --> 00:27:30,030 also along the axon. 461 00:27:30,030 --> 00:27:34,530 And generally when a branch starts to form, it originates 462 00:27:34,530 --> 00:27:36,440 at one of these swellings. 463 00:27:36,440 --> 00:27:39,540 So in other words, something makes the philipodia adhere to 464 00:27:39,540 --> 00:27:43,230 something, adhesive properties are changing. 465 00:27:43,230 --> 00:27:49,730 And this whole axon is capable of growth at that stage and it 466 00:27:49,730 --> 00:27:51,530 can form branches. 467 00:27:51,530 --> 00:27:53,420 And then it starts to arborize. 468 00:27:53,420 --> 00:27:56,610 And when it starts to arborize, it looks like the 469 00:27:56,610 --> 00:27:59,420 second picture here. 470 00:27:59,420 --> 00:28:03,500 It starts forming arbors in a lot of different places, not 471 00:28:03,500 --> 00:28:05,170 everywhere. 472 00:28:05,170 --> 00:28:07,880 But it forms a lot of very rudimentary arbors. 473 00:28:07,880 --> 00:28:09,600 It's not forming big arbors yet. 474 00:28:09,600 --> 00:28:12,690 475 00:28:12,690 --> 00:28:16,310 But this axon here, its final arbor might end 476 00:28:16,310 --> 00:28:18,280 up being right here. 477 00:28:18,280 --> 00:28:20,520 And then it can grow further. 478 00:28:20,520 --> 00:28:24,320 And it will form arbors both beyond that point and proximal 479 00:28:24,320 --> 00:28:27,000 to that point. 480 00:28:27,000 --> 00:28:30,770 And I'm showing that here, the third picture. 481 00:28:30,770 --> 00:28:33,750 It starts to focalize. 482 00:28:33,750 --> 00:28:37,010 Certain arbors start to become augmented 483 00:28:37,010 --> 00:28:40,500 and the others shrivel. 484 00:28:40,500 --> 00:28:44,170 So it loses some of the branches that it's forming. 485 00:28:44,170 --> 00:28:47,195 Its early exuberance is decreased. 486 00:28:47,195 --> 00:28:50,830 It starts increasing is arbors at certain points. 487 00:28:50,830 --> 00:28:52,660 Here's one that's a little further along. 488 00:28:52,660 --> 00:28:56,710 Notice that the one going further here is totally lost. 489 00:28:56,710 --> 00:29:00,040 And it might form abors elsewhere too. 490 00:29:00,040 --> 00:29:04,290 For example, the optic tract that's arborizing in the 491 00:29:04,290 --> 00:29:08,800 tectum of the midbrain could be forming an arbor in part of 492 00:29:08,800 --> 00:29:10,050 the geniculate body. 493 00:29:10,050 --> 00:29:11,860 494 00:29:11,860 --> 00:29:13,850 The very same axon. 495 00:29:13,850 --> 00:29:16,710 Not all of them do that, some of them will terminate only in 496 00:29:16,710 --> 00:29:20,180 the geniculate body or only in the tectum, but quite a few of 497 00:29:20,180 --> 00:29:23,730 them actually terminate in both. 498 00:29:23,730 --> 00:29:26,110 And we have direct experimental evidence for 499 00:29:26,110 --> 00:29:29,280 that, we also have the Golgi evidence. 500 00:29:29,280 --> 00:29:33,470 And then they focalize further. 501 00:29:33,470 --> 00:29:36,710 Initially they distributed throughout from 502 00:29:36,710 --> 00:29:38,280 superficial to deep. 503 00:29:38,280 --> 00:29:42,090 But then they start increasing their arbor only in certain 504 00:29:42,090 --> 00:29:44,840 layers and they lose the extra branches elsewhere. 505 00:29:44,840 --> 00:29:48,350 So that's a laminar focalization. 506 00:29:48,350 --> 00:29:51,230 And that's already happened before the eyes open in the 507 00:29:51,230 --> 00:29:54,370 hamster, and the rat, and the mouse. 508 00:29:54,370 --> 00:29:56,370 But there continues to be changes in the 509 00:29:56,370 --> 00:29:57,705 way that arbor looks. 510 00:29:57,705 --> 00:30:01,030 And just referred to that as maturation. 511 00:30:01,030 --> 00:30:03,445 The appearance of the terminals, especially as scene 512 00:30:03,445 --> 00:30:07,306 in electron microscopy continues to change for a 513 00:30:07,306 --> 00:30:12,110 while as the animal gets older. 514 00:30:12,110 --> 00:30:17,000 And I also point out here that in our studies of brain 515 00:30:17,000 --> 00:30:24,980 damage, it's in this early period that if you make a 516 00:30:24,980 --> 00:30:29,780 lesion there, they can regrow. 517 00:30:29,780 --> 00:30:37,940 If we do the same thing here or here they don't regrow. 518 00:30:37,940 --> 00:30:40,890 It doesn't mean they don't change in the way they're 519 00:30:40,890 --> 00:30:45,320 growing, but it just means they have trouble 520 00:30:45,320 --> 00:30:48,280 regenerating. 521 00:30:48,280 --> 00:30:52,650 Most of them, there's a few of them that will regrow. 522 00:30:52,650 --> 00:30:53,980 Let's say a little bit-- 523 00:30:53,980 --> 00:30:56,580 what time is it here?-- 524 00:30:56,580 --> 00:30:58,760 say a little bit about formation the map now. 525 00:30:58,760 --> 00:31:02,610 I mentioned the chemoaffinity hypothesis, right? 526 00:31:02,610 --> 00:31:05,190 So let's say a little more about that because quite a bit 527 00:31:05,190 --> 00:31:07,380 has been discovered in more recent years. 528 00:31:07,380 --> 00:31:11,210 529 00:31:11,210 --> 00:31:15,500 These types of molecules have been found to be largely 530 00:31:15,500 --> 00:31:20,710 responsible for map formation. 531 00:31:20,710 --> 00:31:25,510 When I say the responsible for map formation, they're 532 00:31:25,510 --> 00:31:29,740 responsible for making the map precise. 533 00:31:29,740 --> 00:31:31,510 It's not that there aren't other mechanisms. 534 00:31:31,510 --> 00:31:34,990 By the time the axons reach the midbrain tectum for 535 00:31:34,990 --> 00:31:39,560 example, the axons representing the upper visual 536 00:31:39,560 --> 00:31:44,310 field are already growing mediately and the axons 537 00:31:44,310 --> 00:31:47,100 representing the lower visual field coming from the upper 538 00:31:47,100 --> 00:31:48,775 retina are already growing laterally. 539 00:31:48,775 --> 00:31:51,870 540 00:31:51,870 --> 00:31:56,740 But in fact, it's not a perfect order. 541 00:31:56,740 --> 00:31:59,620 And as far as the nasal temporal axis goes, those 542 00:31:59,620 --> 00:32:01,865 axons are just mixed in with each other. 543 00:32:01,865 --> 00:32:04,770 You clearly have to have some other mechanism for 544 00:32:04,770 --> 00:32:06,020 sorting them off. 545 00:32:06,020 --> 00:32:09,580 546 00:32:09,580 --> 00:32:12,360 Now Sperry assumed that they went to where they were 547 00:32:12,360 --> 00:32:16,420 attracted, almost like a lock and key. 548 00:32:16,420 --> 00:32:18,660 But here's what was discovered. 549 00:32:18,660 --> 00:32:24,510 In the retina you have the EPH receptors. 550 00:32:24,510 --> 00:32:28,730 And the ligand and for those receptors are the Ephrins. 551 00:32:28,730 --> 00:32:32,330 And the Ephrin ligands are found in the tectum or 552 00:32:32,330 --> 00:32:33,900 superior colliculus. 553 00:32:33,900 --> 00:32:38,310 So for example, we only need to deal with one of them here. 554 00:32:38,310 --> 00:32:40,410 Let's deal with that picture there. 555 00:32:40,410 --> 00:32:42,300 First of all, here you see the retina. 556 00:32:42,300 --> 00:32:45,290 And this is how we know it will form the adult axons from 557 00:32:45,290 --> 00:32:46,920 the temporal retina will terminate 558 00:32:46,920 --> 00:32:48,480 anteriorally in the tectum. 559 00:32:48,480 --> 00:32:53,250 those from the nasal retina will terminate posteriorly. 560 00:32:53,250 --> 00:32:55,530 We also know that when they initially grew into the tectum 561 00:32:55,530 --> 00:32:58,460 they weren't distributed so precisely. 562 00:32:58,460 --> 00:33:02,930 OK, now if we look at Ephrin-A3 A3, we see that it's 563 00:33:02,930 --> 00:33:06,840 concentrated in the temporal retina, almost none of it in 564 00:33:06,840 --> 00:33:07,480 the nasal retina. 565 00:33:07,480 --> 00:33:11,540 It's distributed in a gradient like way across the retina. 566 00:33:11,540 --> 00:33:15,990 In Ephrin-A2, the ligand is concentrated 567 00:33:15,990 --> 00:33:19,210 in the caudal tectum. 568 00:33:19,210 --> 00:33:23,260 It's not found in the rostrum or the tectum and its levels 569 00:33:23,260 --> 00:33:24,545 of that molecule are in between. 570 00:33:24,545 --> 00:33:27,860 571 00:33:27,860 --> 00:33:38,176 Now it turns out that when you have Ephrin-A3, it's repelled. 572 00:33:38,176 --> 00:33:41,680 573 00:33:41,680 --> 00:33:46,710 The EPHA3 is repelled by Ephrin-A2. 574 00:33:46,710 --> 00:33:51,570 So those axons won't grow into the caudal tectum. 575 00:33:51,570 --> 00:34:00,240 If it doesn't have or it has very little EPH-A3, it can 576 00:34:00,240 --> 00:34:02,800 grow right through the whole tectum. 577 00:34:02,800 --> 00:34:04,366 And here's how that was demonstrated. 578 00:34:04,366 --> 00:34:07,500 579 00:34:07,500 --> 00:34:10,949 What they've done is taken along little tracks here in 580 00:34:10,949 --> 00:34:15,960 tissue culture, they've gotten the axons to grow. 581 00:34:15,960 --> 00:34:21,250 These came from the temporal part of the retina, these came 582 00:34:21,250 --> 00:34:23,270 from the nasal part of the retina. 583 00:34:23,270 --> 00:34:24,860 And they're going down these tracks. 584 00:34:24,860 --> 00:34:28,770 585 00:34:28,770 --> 00:34:30,540 What are they growing on? 586 00:34:30,540 --> 00:34:33,250 They're growing on tectal membranes. 587 00:34:33,250 --> 00:34:37,760 And they mince up the membranes, so they get this 588 00:34:37,760 --> 00:34:41,290 membrane preparation, all little pieces of membrane. 589 00:34:41,290 --> 00:34:48,779 And they get those from purely anterior tectum and then more 590 00:34:48,779 --> 00:34:51,100 and more posterior tectum. 591 00:34:51,100 --> 00:34:53,940 And they lay them down in these tacks. 592 00:34:53,940 --> 00:34:55,870 Whoops, what did I just do? 593 00:34:55,870 --> 00:35:03,350 594 00:35:03,350 --> 00:35:04,600 Sorry about that. 595 00:35:04,600 --> 00:35:07,020 596 00:35:07,020 --> 00:35:11,030 Notice that if they're coming from temporal retina cells 597 00:35:11,030 --> 00:35:15,010 they get stopped as soon as they start encountering 598 00:35:15,010 --> 00:35:18,510 membranes from the posterior tectum. 599 00:35:18,510 --> 00:35:21,910 If they're coming from nasal retina, it doesn't matter 600 00:35:21,910 --> 00:35:25,420 whether they're encountering membranes from the anterior or 601 00:35:25,420 --> 00:35:28,030 from the posterior tectum, they can keep growing. 602 00:35:28,030 --> 00:35:32,140 So that shows that the mechanism of the EPH receptors 603 00:35:32,140 --> 00:35:35,540 interacting with the Ephrins is a repulsive mechanism, it's 604 00:35:35,540 --> 00:35:38,510 a repelling effect. 605 00:35:38,510 --> 00:35:43,500 And that's how these gradients of these molecules are 606 00:35:43,500 --> 00:35:47,520 governing the distribution of the axons. 607 00:35:47,520 --> 00:35:51,350 Later they discovered that there were gradients in other 608 00:35:51,350 --> 00:35:52,690 redirection as well. 609 00:35:52,690 --> 00:35:56,160 So even though the axons are already organized, they from a 610 00:35:56,160 --> 00:35:58,760 better map with the help of these molecules. 611 00:35:58,760 --> 00:36:01,050 That was discovered by these two different groups, one at 612 00:36:01,050 --> 00:36:05,260 Harvard, Flanagan, and one in Germany, the [UNINTELLIGIBLE] 613 00:36:05,260 --> 00:36:06,510 group. 614 00:36:06,510 --> 00:36:10,020 615 00:36:10,020 --> 00:36:15,860 So with all these chemicals it's all pretty rigid, right? 616 00:36:15,860 --> 00:36:17,366 Well, not quite. 617 00:36:17,366 --> 00:36:20,140 618 00:36:20,140 --> 00:36:26,270 Let's take a developing colliculus 619 00:36:26,270 --> 00:36:31,710 here of a baby hamster. 620 00:36:31,710 --> 00:36:34,210 And here comes the optic tract. 621 00:36:34,210 --> 00:36:36,040 Here are the axons coming in. 622 00:36:36,040 --> 00:36:39,560 623 00:36:39,560 --> 00:36:43,530 And right while those axons are growing in, this 624 00:36:43,530 --> 00:36:46,730 is what we'll do. 625 00:36:46,730 --> 00:36:52,650 We'll use a cautery and we'll burn off the 626 00:36:52,650 --> 00:36:53,900 whole caudal tectum. 627 00:36:53,900 --> 00:36:59,520 628 00:36:59,520 --> 00:37:01,870 What does that do? 629 00:37:01,870 --> 00:37:07,750 What happens is we'll us retinal coordinates. 630 00:37:07,750 --> 00:37:11,510 The temporal retina axons will terminate there, which they 631 00:37:11,510 --> 00:37:13,092 normally do. 632 00:37:13,092 --> 00:37:18,140 But the axons from the nasal retina that would normally 633 00:37:18,140 --> 00:37:24,650 have gone back here, terminate there. 634 00:37:24,650 --> 00:37:28,900 So the whole map ends up in crest in 635 00:37:28,900 --> 00:37:30,150 that remaining tectum. 636 00:37:30,150 --> 00:37:33,060 637 00:37:33,060 --> 00:37:36,460 The superior retina is represented here, the inferior 638 00:37:36,460 --> 00:37:38,380 retina here. 639 00:37:38,380 --> 00:37:39,680 That's normal. 640 00:37:39,680 --> 00:37:41,370 And now the whole map is represented 641 00:37:41,370 --> 00:37:43,930 in the smaller space. 642 00:37:43,930 --> 00:37:45,315 That's called map compression. 643 00:37:45,315 --> 00:37:48,490 644 00:37:48,490 --> 00:37:50,790 So what's happened to these molecules? 645 00:37:50,790 --> 00:37:55,275 It shows that it's not a lock and key kind of effect at all, 646 00:37:55,275 --> 00:37:57,340 it's due tot he gradients in molecules. 647 00:37:57,340 --> 00:38:00,500 648 00:38:00,500 --> 00:38:05,370 And that's been verified by effects of expansion. 649 00:38:05,370 --> 00:38:08,310 That's compression here. 650 00:38:08,310 --> 00:38:10,170 You also get map expansion. 651 00:38:10,170 --> 00:38:13,640 How would you test that? 652 00:38:13,640 --> 00:38:17,140 What's the experiment you'd want to do to test to see if 653 00:38:17,140 --> 00:38:18,390 the map can also expand? 654 00:38:18,390 --> 00:38:21,350 655 00:38:21,350 --> 00:38:25,300 Well you could make the tectum bigger, but it's a lot easier 656 00:38:25,300 --> 00:38:29,660 to just ablate part of the retina. 657 00:38:29,660 --> 00:38:34,520 So one of my graduate students made holes in the retina. 658 00:38:34,520 --> 00:38:39,550 Again, with burn lesions in newborn animals, taking out 659 00:38:39,550 --> 00:38:41,046 little pieces of retina. 660 00:38:41,046 --> 00:38:45,150 661 00:38:45,150 --> 00:38:51,200 And if you made a hole in the retina, if you did that in the 662 00:38:51,200 --> 00:38:54,630 adult, you end up with a hole in the map. 663 00:38:54,630 --> 00:38:58,270 Axons are just not found in one part of the tectum. 664 00:38:58,270 --> 00:39:01,730 If you did that in the baby, they filled in. 665 00:39:01,730 --> 00:39:05,190 666 00:39:05,190 --> 00:39:09,080 The remaining map filled in the gap, 667 00:39:09,080 --> 00:39:11,230 so the map was expanding. 668 00:39:11,230 --> 00:39:15,330 And this has been done in goldfish in regeneration 669 00:39:15,330 --> 00:39:19,960 situations in adult goldfish. 670 00:39:19,960 --> 00:39:23,190 When you take off much of one half of the retina, the 671 00:39:23,190 --> 00:39:27,310 remaining will expand across the tectum. 672 00:39:27,310 --> 00:39:28,960 So he was getting things like that 673 00:39:28,960 --> 00:39:30,910 happening in the baby hamster. 674 00:39:30,910 --> 00:39:34,050 675 00:39:34,050 --> 00:39:36,090 You get both compression and expansion. 676 00:39:36,090 --> 00:39:39,580 That we call plasticity of the developing maps. 677 00:39:39,580 --> 00:39:42,470 678 00:39:42,470 --> 00:39:48,280 To explain what was going on, we refer to experiments on 679 00:39:48,280 --> 00:39:50,080 collateral sprouting. 680 00:39:50,080 --> 00:39:52,080 Have we talk about the collateral sprouting at all? 681 00:39:52,080 --> 00:39:54,820 682 00:39:54,820 --> 00:39:56,110 How many of you remember something 683 00:39:56,110 --> 00:39:57,870 about collateral sprouting? 684 00:39:57,870 --> 00:40:01,220 Let's go through collateral sprouting experiments that 685 00:40:01,220 --> 00:40:04,040 were done now not in the adult, which we probably 686 00:40:04,040 --> 00:40:08,370 mentioned before, but during development. 687 00:40:08,370 --> 00:40:11,880 This is a side view of the hamster brain stem, very much 688 00:40:11,880 --> 00:40:14,830 like a rat, or a mouse, or a developing human. 689 00:40:14,830 --> 00:40:17,870 These are optic tract axons. 690 00:40:17,870 --> 00:40:20,860 And this is the normal route for a one bundle of optic 691 00:40:20,860 --> 00:40:24,650 tract axons coming from the optic chiasm up the side of a 692 00:40:24,650 --> 00:40:29,480 diencephalon, terminations in the geniculate bodies, ventral 693 00:40:29,480 --> 00:40:31,750 and dorsal, and in the superior 694 00:40:31,750 --> 00:40:33,900 colliculus or optic tectum. 695 00:40:33,900 --> 00:40:37,330 There are other pathways coming from neurons there, the 696 00:40:37,330 --> 00:40:40,300 very neurons that are receiving these terminations, 697 00:40:40,300 --> 00:40:46,140 that project forward both to the geniculate bodies, 698 00:40:46,140 --> 00:40:49,430 especially to the ventral one, and to that nucleus sitting 699 00:40:49,430 --> 00:40:52,510 right next to the geniculate body, LP, the lateral 700 00:40:52,510 --> 00:40:55,010 posterior nucleus. 701 00:40:55,010 --> 00:40:56,580 So now here's what we'll do. 702 00:40:56,580 --> 00:41:01,490 We'll take our lesion maker and we'll just ablate the 703 00:41:01,490 --> 00:41:02,740 superior colliculus. 704 00:41:02,740 --> 00:41:05,450 705 00:41:05,450 --> 00:41:08,340 And then we'll wait, we'll let the animal grow up and see 706 00:41:08,340 --> 00:41:11,130 what's happened to the optic tract. 707 00:41:11,130 --> 00:41:15,000 What happens is that this normal 708 00:41:15,000 --> 00:41:18,370 projection here is now missing. 709 00:41:18,370 --> 00:41:22,450 But instead, the axons grow into what's left in deeper 710 00:41:22,450 --> 00:41:25,740 layers that normally don't get a retinal projection. 711 00:41:25,740 --> 00:41:28,590 They sprout in there and form some terminations. 712 00:41:28,590 --> 00:41:33,670 It's not of normal size, but it's clearly there. 713 00:41:33,670 --> 00:41:36,185 But this projection is now missing. 714 00:41:36,185 --> 00:41:39,590 715 00:41:39,590 --> 00:41:44,050 The retinal projection will sprout there in both of these 716 00:41:44,050 --> 00:41:48,050 places where the colliculus normally projects. 717 00:41:48,050 --> 00:41:51,250 It's like they're filling in some of the terminal space 718 00:41:51,250 --> 00:41:52,610 that was made available. 719 00:41:52,610 --> 00:41:55,920 So now the retina ends up projecting more densely than 720 00:41:55,920 --> 00:42:00,170 normal in these two places in the thalamus. 721 00:42:00,170 --> 00:42:04,160 And here's a new projection in the midbrain. 722 00:42:04,160 --> 00:42:06,490 They're showing a type of collateral sprouting. 723 00:42:06,490 --> 00:42:09,990 724 00:42:09,990 --> 00:42:13,870 Well the inferior colliculus normally projects there to the 725 00:42:13,870 --> 00:42:14,970 medial geniculate body. 726 00:42:14,970 --> 00:42:19,580 What if we get rid of that projection and we get rid of 727 00:42:19,580 --> 00:42:20,830 the tectum? 728 00:42:20,830 --> 00:42:22,950 729 00:42:22,950 --> 00:42:27,690 Well what happens is that the retinal axons now will sprout 730 00:42:27,690 --> 00:42:32,770 not only in the LP and in the LGB there, they will actually 731 00:42:32,770 --> 00:42:35,530 sprout right into the auditory thalamus. 732 00:42:35,530 --> 00:42:39,000 So now the retina projects into the auditory system. 733 00:42:39,000 --> 00:42:42,540 And now the, in this case, hamster, it happens in the 734 00:42:42,540 --> 00:42:46,085 ferret, the rat also, most of the experiments have been done 735 00:42:46,085 --> 00:42:50,190 with hamster and ferret, you get the retina projecting not 736 00:42:50,190 --> 00:42:53,250 only to the later geniculate body which projects to the 737 00:42:53,250 --> 00:42:56,230 visual cortex, but also projects to the medial 738 00:42:56,230 --> 00:42:59,360 geniculate body which projects to the auditory cortex. 739 00:42:59,360 --> 00:43:01,160 So now you have an extra visual cortex. 740 00:43:01,160 --> 00:43:03,810 741 00:43:03,810 --> 00:43:07,510 In the absence of normal auditory input to the auditory 742 00:43:07,510 --> 00:43:08,990 cortex, now it's getting retinal input. 743 00:43:08,990 --> 00:43:11,680 744 00:43:11,680 --> 00:43:14,560 And there has been some data collected in Mriganka Sur's 745 00:43:14,560 --> 00:43:19,710 lab, behavioral evidence, that in fact that's a functional 746 00:43:19,710 --> 00:43:20,270 projection. 747 00:43:20,270 --> 00:43:23,510 He's done very nice electrophysiology of that 748 00:43:23,510 --> 00:43:24,390 projection. 749 00:43:24,390 --> 00:43:26,310 He's also done some behavioral work. 750 00:43:26,310 --> 00:43:30,930 751 00:43:30,930 --> 00:43:33,600 So what's happening here? 752 00:43:33,600 --> 00:43:34,740 This is theoretical. 753 00:43:34,740 --> 00:43:40,440 If I take two axons coming into a structure like this 754 00:43:40,440 --> 00:43:47,400 superficial tectum and they share the space like this and 755 00:43:47,400 --> 00:43:50,740 we get rid of one of them, we'll get rid of that one. 756 00:43:50,740 --> 00:43:55,440 So this will all degenerate here. 757 00:43:55,440 --> 00:43:57,590 The remaining axon will spread. 758 00:43:57,590 --> 00:44:01,690 It won't form an arbor that's bigger than normal, but it 759 00:44:01,690 --> 00:44:05,490 will spread over a larger area, as if it's being held in 760 00:44:05,490 --> 00:44:08,220 place by the competition. 761 00:44:08,220 --> 00:44:11,230 And there are many experiments like that, that show that 762 00:44:11,230 --> 00:44:14,390 axons compete in some way. 763 00:44:14,390 --> 00:44:18,060 And competition, we think now, can take several forms. 764 00:44:18,060 --> 00:44:21,050 They can be competing for the terminal space, either because 765 00:44:21,050 --> 00:44:23,170 of the growth factors, they get there, and there's good 766 00:44:23,170 --> 00:44:25,700 evidence for that. 767 00:44:25,700 --> 00:44:28,070 Also we looked with the electron microscope, they 768 00:44:28,070 --> 00:44:33,400 actually compete to occupy the synaptic sites. 769 00:44:33,400 --> 00:44:37,087 In many cases, more than one axon can occupy 770 00:44:37,087 --> 00:44:38,760 the synaptic sites. 771 00:44:38,760 --> 00:44:41,820 But we also know about these contact reactions that have 772 00:44:41,820 --> 00:44:46,400 been seen in tissue culture, that axons will space each 773 00:44:46,400 --> 00:44:48,930 other out because they're affected by each other, they 774 00:44:48,930 --> 00:44:50,890 retract from each other. 775 00:44:50,890 --> 00:44:55,590 That's a kind of competition too, this contact inhibition 776 00:44:55,590 --> 00:44:57,340 of extension it's sometimes called. 777 00:44:57,340 --> 00:45:01,650 778 00:45:01,650 --> 00:45:07,820 OK, so next time I'm going to talk about the ability of 779 00:45:07,820 --> 00:45:11,333 axons to compete and how you can change their competitive 780 00:45:11,333 --> 00:45:11,783 growth vigor by chemical means, by activity, and by 781 00:45:11,783 --> 00:45:11,920 lesions, or pruning effects. 782 00:45:11,920 --> 00:45:16,170 We'll go through those at the beginning of the next hour.